A method of transforming an image format of image data stored in an apparatus connected via a network is disclosed. The method includes the steps of providing the apparatus with format information containing image formats of image data that can be transformed, receiving a request for transforming the image format of the image data from the apparatus, acquiring the image data from the apparatus, transforming the image format of the image data acquired from the apparatus thereby to form format-transformed image data, and providing the format-transformed image data to the apparatus. According to the above arrangements, an image transforming apparatus using the method can transform the image format of the image data on behalf of the apparatus.